Genetic modification of insects as pest control part 3

posted in: Science | 9

In part 1 of this series, I explained how we’ve been using genetic engineering of sorts for nearly half a century to control insects by using radiation to induce sterility or other dominant lethal mutations in insects. In part 2, I explained how we can use genetic engineering to make these projects safer and easier.

So… part 3. What’s the next step? Put it to the test!

A system is only good if we can actually use it. We can test stuff in a laboratory and under cages all we want, but the tests only matter if they have an effect on real-world populations. This step has now been taken. Mosquitoes which have been modified by the Oxford-based company Oxitec to carry tetracycline repressible lethal genes (as described in part 2) are being released into the wild to combat outbreaks of Dengue fever in the Cayman Islands (see press release). This real-world test will help us to see if this technique will work in real-world situations.

It's kind of pretty in a 'they're gonna kill us all' kind of way. Image via Wikimedia.

Dengue is largely spread by Aedes aegypti mosquitoes which feed pretty much only on humans and are present in the highest populations around our cities…they’re mosquitoes which have adapted to an urban lifestyle. They bite during the day, so we can’t use bed nets. They’re notoriously resistant to many insect repellents… and they’re evolving resistance to insecticides used in vector control programs. Carbamates, pyrethroids… even DDT shows resistance in many populations of these guys. We simply need new tools.

Thanks to Medfly research, we now have new tools. One of these new tools, the RIDL genes explained in part two, is what is currently being tested in the Cayman Islands. Genetically modified Aedes aegypti have been released in an effort to bring the mosquito populations down past the number where they can spread disease. They’re only releasing male mosquitoes which don’t feed on blood, so they can’t transmit the disease to humans.

An Aedes aegypti larva. Image via Wikimedia.

There’s an even cooler ecological quirk to this technique, though. The lethal protein takes awhile to build up in the mosquito larvae, which means the mosquito larvae take a while to die. Before they die, they actually compete with wild type larvae for resources which should help to keep populations from quickly rebounding. The larvae of Aedes aegypti live in water where they feed on detritus. They don’t feed on blood and are unable to transmit disease.

In an AP article titled Mutant mosquitoes fight dengue in Cayman Islands, Oxitec is predicting an 80% reduction in Aedes mosquitoes from their test release of modified mosquitoes. I’m muting my enthusiasm until I see some data from this test as well as an decrease in Dengue transmission. It’s one thing to release the mosquitoes with a prediction the populations will fall, but ultimately I’d still like to see proof that this will work on this system. This is a good step in the right direction, though.

However, the program is meeting a lot of resistance from many including anti-GMO groups. From the AP article:

“If we remove an insect like the mosquito from the ecosystem, we don’t know what the impact will be,” said Pete Riley, campaign director of GM Freeze, a British non-profit group that opposes genetic modification.

He said mosquito larvae might be food for other species, which could starve if the larvae disappear. Or taking out adult mosquito predators might open up a slot for other insect species to slide in, potentially introducing new diseases.

Human ecology is a weird, wonderful thing. We bring in all sorts of animals wherever we go from dogs to birds to rats, and there are urban mosquitoes which specialize on all of these. RIDL relies on species-specific patterns of mating and reproduction, so we can target it pretty effectively. If the Aedes aegypti mosquitoes die out, something else will take its place and Riley is correct on this. It could be something like Aedes albopictus which also spreads Dengue or Culex quinquefasciatus which also spreads disease (not Dengue, though) but the mosquitoes could also be potentially be replaced by something which feeds on birds and rarely bites humans. We won’t know until this happens because nobody can predict the future.

Of course, because there are mosquito species other than Aedes aegypti those poor mossie predators will be OK because that will be the only species affected by the program. Either way, though… Riley’s arguing against mosquito control rather than the genetic modification of mosquitoes as a tool. I think this is weird because, vector borne diseases kill millions of people across the globe every year. I think it’s simply a good idea to try to eliminate the disease.

The next statement I found more than a bit odd because Riley seems to be using the concept of accidental species introduction or removal as an argument against pest control:

Humans have a patchy track record of interfering with natural ecosystems, Riley said. In the past, such interventions have led to the overpopulation of species including rabbits and deer. “Nature often does just fine controlling its problems until we come along and blunder into it.”

More than half the world’s population is at risk for dengue and over 50 million cases occur per year around the world. I’m not sure why he’s concluding nature’s doing just fine on this one because from the perspective of dengue sufferers… nature’s doing a crappy job managing this problem.

We have a patchy track record when it comes to ecology, and we should take it for exactly what it is. There are both successes and failures embedded in our history, even in mosquito control. The point is that we learn from both our failures and our successes. If something doesn’t work, we try again with a new technique. The Sterile Insect Technique has been a smashing success for all pests that it has been used for so far, and this is merely a re-invention of the technique which is an improvement over the original.

Consider this, though… the main reason the Panama Canal took two attempts to finish was because of yellow fever. We eventually figured out that if we eliminate mosquitoes from our living areas, yellow fever cases dropped. We were then able to finish the Panama Canal, and today it’s one of the main hubs for international trade routes.

This problem did not ‘work itself out’. Nature did not ‘do just fine controlling the problem’. We did this… we triumphed over the hostile forces of nature.

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Joe Ballenger is an entomologist who works in the biotech industry as a contractor. In his spare time, he helps answer questions about bugs at Ask an Entomologist. https://askentomologists.wordpress.com/